1. Field of the Invention
This invention is concerned with the manufacture of light olefin hydrocarbons from lower alcohols or their ethers. It is particularly concerned with the catalytic conversion of such alcohols and ethers selectively to a hydrocarbon mixture characterized by a predominance of olefins. This invention further provides a novel method for converting methanol and/or dimethyl ether to low durene-content gasoline or gasoline blending stocks as well as higher boiling distillates and lubes.
2. Description of the Prior Art
A remarkable growth in the production of synthetic fibers, plastics and rubber has taken place in recent decades. This growth, to a very large extent, has been supported and encouraged by an expanding supply of inexpensive petrochemical raw materials such as ethylene, propylene, and other four and five carbon olefins. Side by side with this remarkable devlopment, there has been an increasing demand for alkylate, made by reacting olefins with isobutane, for use as a high octane gasoline component. Environmental factors which limit the lead content of gasoline are likely to aggravate the need for alkylate.
Burgeoning demand for olefins, particularly ethylene, propylene and butenes, has of couse led to periods of shortage, either due to short supply of suitable feedstocks or to limited processing capacities. In any case, it would appear desirable to provide efficient means for converting raw materials other than petroleum to olefins.
In recent years, the patent art has disclosed that methanol and/or dimethyl ether, which may be obtained from coal, natural gas or biomass, can be converted to more complex hydrocarbons, such as olefins and aromatics, by utilizing a novel group of zeolites, exemplified by ZSM-5 zeolites. The C.sub.2 -C.sub.4 olefinic hydrocarbons may be obtained in this catalytic conversion. The reaction is hightly exothermic and the olefins initially formed have a tendency to undergo further reaction to product aromatic hydrocarbons useful in the production of motor gasoline. A large body of this patent art is concerned with various aspects of selectively converting methanol and/or dimethyl ether to light olefins.
The production of olefins from aliphatic ethers by catalytic conversion with an HZSM-5 zeolite catalyst is disclosed in U.S. Pat. No. 3,894,106 of Chang et al. U.S. Pat. No. 4,025,572 of Lago discloses that ethylene selectivity can be improved by diluting ZSM-5 with an inert diluent while a similar result is achieved, according to U.S. Pat. No. 4,025,575 of Chang et al, through use of subatmospheric partial pressure of the feed. Selectivity with ethylene is also improved by employing a ZSM-5 zeolite in large crystal form of at least one micron either alone (U.S. Pat. No. 4,025,571 of Lago) or in combination with added metals (U.S. Pat. No. 4,148,835 of Chang et al). Better selectivity is also obtained by interdispersing amorphous silica within the interior of the crystalline structure of the zeolite catalyst such as disclosed in U.S. Pat. Nos. 4,060,568 and 4,100,219 of Rodewald.
The conversion of methanol and/or dimethyl ether to gasoline is another important area of technology which has the potential of becoming even more important as the supply of crude oil is diminished and/or increased in price. Particularly, an advantageous catalyst which is utilized in the conversion of methanol to gasoline is a special class of crystalline aluminosilicate zeolite catalysts of which HZSM-5 is the most preferred member. There are many patents and publications which describe the conversion of methanol to gasoline over special zeolites, including U.S. Pat. Nos. 3,931,349; 3,969,426; 3,899,544; 3,894,104; 3,940,916; and 3,894,102; the disclosures of which are incorporated by reference.
One particular problem residing in the conversion of methanol to gasoline over ZSM-5 type zeolites is that durene is produced in amounts higher than that expected from C.sub.10 aromatic equilibrium distributions. Once an aromatic ring is formed in the presence of unreacted methanol, alkylation to tetramethyl benzenes occurs rapidly, but the smaller higher melting durene molecule (1,2,4,5-tetramethyl benzene, melting point 175.degree. F.) diffuses out the ZSM-5 pore much more rapidly than isodurene (1,2,3,5-tetramethyl benzene) or prehnitene (1,2,3,4-tetramethyl benzene). Durene is an undesirable component of gasoline because of its high melting point. Thus, durene has a tendency to crystallize out of solution at temperatures below 175.degree. F. and deposit within the carburetor or intake manifold of an automobile resulting in an unsatisfactory performance. Based on the results of vehicle testing, an upper limit of 4 wt. % durene in the gasoline is considered acceptable for general motor usage.
There have been various proposals advanced in order to control or minimize the amount of durene which is produced in the catalytic conversion of methanol to gasoline. One proposal has suggested the isomerization of the bottoms fraction of a methanol to gasoline process in order to decrease the durene content. Specifically, U.S. Pat. No. 4,304,951 discloses a process for decreasing the durene content of the bottoms fraction obtained from the catalytic conversion of methanol to gasoline comprising contacting the bottoms fraction with hydrogen at elevated temperatures and pressures over a supported hydrogenation metal catalyst for a period of time sufficient to decrease the durene content and enhance the production of distillate.
U.S. Pat. No. 4,025,576 discloses a process for converting methanol and/or dimethyl ether to low durene-content gasoline or gasoline blending stocks by catalytic contact of the feed at subatmospheric partial pressure with ZSM-5 type catalysts. A multi-stage method for producing gasoline from a methanol feed comprises contacting the feed at a subatmospheric inlet partial pressure of the feed with a first stage ZSM-5 type catalyst at a temperature of 600.degree. F. to 900.degree. F. and thereby convert the methanol to a first stage conversion product comprising steam and light olefin hydrocarbons and contacting in a subsequent stage the first stage conversion product with a subsequent ZSM-5 catalyst at temperatures up to about 1290.degree. F. thereby forming a gasoline product containing low durene contents not above about 2 wt. %.
It is further known that the production of durene can be reduced by operating the methanol-to-gasoline conversion reactor at elevated temperatures. For example, a fixed-bed process with a reactor temperature greater than 850.degree. F., a 3.2 WHSV, 300 psig, 4.5 recycle ratio has yielded high gasoline selectivities, e.g., greater than 85% of the hydrocarbons produced including alkylate, with durene levels at less than 3.5 wt. %. However, at these reaction conditions, a high partial pressure of water is formed. The high partial pressure of water combined with the elevated temperature resulted in rapid permanent catalyst deactivation due to steaming. Operating at temperatures less than 800.degree. F., on the other hand, resulted in satisfactory catalyst life but yielded durene concentrations in the gasoline product of greater than 4 wt. %.
It is also known to convert methanol to olefins over ZSM-5 as discussed above and to subsequently convert the olefins over ZSM-5 to higher olefins, an olefinic distillate and/or to hydrocarbons boiling in the lube oil range.